1. Field of the Invention
The present invention relates to an apparatus and method for cooling an electrical submersible pump. More specifically, the invention relates to cooling the motor of an electrical submersible pump by drawing fluid from a wellbore sump along the motor.
2. Description of the Related Art
Electrical submersible pumps (“ESP”) are used to pump fluids up from a wellbore. Wellbore fluids may include oil, natural water, or water drive fluid. Water drive fluid is fluid that is injected into a rock formation under pressure and is used to push, or drive, minerals such as oil or gas towards a wellbore. The water drive fluid enters the wellbore along with the minerals and must be pumped out with the minerals.
The motor used to drive the ESP pump generates heat and thus the motor must be cooled to prolong the life of the motor. Because the ESP is generally submerged in fluid in the wellbore, one method of cooling the motor is to transfer heat from the motor to the fluid surrounding the motor. Heat transfer from the motor to the surrounding fluid is more efficient when fluid is flowing across the outside of the motor housing. The pump, which is located above the motor in the wellbore, can be used to draw wellbore fluid up from below the motor, along the motor housing, and into the pump inlet. In some conditions, the fluid surrounding the motor remains static, resulting in poor heat transfer.
One such condition may occur with a horizontal well in a gassy formation. The ESP may be used to dewater the formation or simply pump wellbore fluids up to the surface. Though used in a gassy formation, ESPs may not be able to handle high concentrations of gas or pockets of gas. Therefore, the ESP may be located in a sump below the horizontal well to avoid any gas pockets that may form. A sump is a branch of the wellbore drilled at an angle off of the horizontal wellbore. The sump allows for a natural separation of the fluids, providing an area for the liquid to flow down to and be produced by the ESP while the gas continues to rise up the annulus of the well. The sump may also have perforations for fluid to directly enter the sump.
The fluid in the sump may not have adequate flow to cool the ESP motor. Fluid enters the sump from two directions—down from the horizontal wellbore and up from perforations in the bottom of the sump. If the pressure from the horizontal wellbore is higher than the pressure from the perforations in the bottom of the sump, the majority of the fluid flowing to the pump inlet is coming from above the pump. The motor, being located below the pump, sits in stagnant fluid. Heat transfer to stagnant fluid is less efficient, resulting in overheating of the pump motor.